The petroleum, chemical, and cement industries, among others, often require the transport of slurries (solid rich liquids) as part of their process handling. Particularly when these slurry pump systems must operate at higher pressures a number of design and maintenance issues arise. Some example pumps that can handle slurries are—piston (e.g., triplex), plunger, centrifugal, diaphragm, displacement pot and progressing cavity (eg. Moyno®) types. They are driven by hydraulic (pressure) and mechanical (mostly with a power transmission rod connected to a crankshaft) means. Any of these means can be powered by a number of prime mover types (electric motor, gasoline engine, natural gas engine, etc . . . ). Only the plunger and piston positive displacement pumps and the batch displacement pot types can handle the higher-pressure needs of industry.
The problem in pumping slurries is that slurries are very erosive of the pump internal parts, especially on valves, seats, plunger, cylinders, pump heads and wherever the slurry flow direction changes and/or the slurry velocity is high, e.g. when in turbulence. The high velocities and rapid flow direction changes in a centrifugal pump, plus their inherent inefficiencies, makes centrifugal type pumps not the first choice for such high-pressure slurry applications. Progressing cavity type pumps can handle the solids content but cannot easily achieve the higher pressures desired due to the elastomer materials in the stator or pump.
The DIAjet, a batch displacement pot type by BHR, is currently available for high pressure slurry pumping. It utilizes pressurized clean fluid with a separate pump (of any type, triplex is most common) that is then pumped into a pressure pot that contains a pre-mixed batch of slurry which is then displaced and discharged from the pot. Production or continuous slurry pumping is difficult with this batch type system, since several pots are needed and they have to be alternately restocked with slurry and resealed for use.
But problems exist in pumping slurries with a positive displacement plunger or piston pump. In addition to the high velocity erosive nature of slurries, especially when flow direction changes, valving is also a problem. As a valve (inlet or discharge of type ball, flute, flapper, or other) closes, the area remaining for flow decreases and the slurry velocity increases (if rate stays the same) which increases the erosive ability of the slurry. Also a hardened steel valve closing onto a hardened steel seat with solids in between makes sealing difficult and results in damaged parts and/or lower efficiencies. The interfering solid particles can be crushed, if they are not too hard, still causing damage to valves and seats. At higher pressures and harder solid particles such interference becomes very damaging. Ceramic valves in these conditions could shatter quickly. Also, rapid velocity or flow pattern changes, as through valves seats, increases the rapid erosion wear of internal pump parts.
Another problem in all slurry pumps is when fluid motion stops and the solids fall out of the carrier fluid. Cleaning out such solids out of the pump is a problem and requires considerable work. If the solid particles would settle and congregate near the fluid end, it would allow easier cleanout of the pump and resumption of pumping.
A number of investigators have tried to address the problems of abrasive materials plugging or eroding cylinders, plungers, pistons and seals. Examples of this can be found in U.S. Pat. No. 3,104,619 to Swarthout, U.S. Pat. No. 4,023,469 to Miller, U.S. Pat. No. 4,157,057 to Bailey, U.S. Pat. Nos. 4,691,620, 4,598,630, and 4,476,771 to Kao. These investigators have developed a number of variations of flushing methods for rings and seals to keep them as free as possible of abrasive materials for longer effective operating lives.
U.S. Pat. No. 7,118,349 (Oglesby), issued to the inventor of this application, addressed the issues that are especially pertinent to a piston pump and defined a pump assembly and a method for maintaining clean fluids in the vicinity of suction and discharge valves of piston pumps.
As mentioned in a previous paragraph a plunger type of pump can also handle the higher-pressure needs of industry. A plunger type of pump can also face the problems described above related to the highly erosive nature of slurries. In a plunger pump, a volume of fluid in the cylinder is displaced by the plunger movement into the cylinder that pressurizes and expels the fluid out the discharge valve. The plunger is stroked axially through the cylinder to provide fluid inlet (as the plunger is withdrawn) and exit (as the plunger is re inserted). Unlike the piston pump, the plunger does not contact the cylinder wall at any time. A non-moving seal mechanism is connected to the cylinder at the base of the plunger and contains pressure and fluids by rings, rubber elements, ceramic elements and other packing materials. The plunger is driven by any number of means—crankshaft, power rod, cam and those are powered, in turn, by any number of prime movers. The cylinder can be made of any number of metals to contain the pressure and fluids. The plunger can be any number of metals or ceramics. The head and valves can be made of any number of metals and ceramics.
There is a need then for significant improvements in the methods and apparatus for plunger type pumps used in slurry service, particularly in addressing the problems of abrasive materials and how to keep the abrasive materials away from the seal mechanism, cylinder, plungers, suction and discharge valves of the slurry plunger pump for improved operation and longer operating life.